An optical fiber (or fiber), i.e., a glass fiber typically surrounded by one or more coating layers, conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the optical cladding ng (i.e., nc>ng).
For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “alpha” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, an alpha-gradient, or a triangle. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
Generally speaking, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber. In a single-mode optical fiber, the signal propagates in a fundamental LP01 mode that is guided in the fiber core, while the higher order modes (e.g., the LP11 mode) are strongly attenuated. The typical diameter of a single-mode or multimode glass fiber is 125 microns. The core of a multimode optical fiber typically has a diameter of between about 50 microns and 62.5 microns, whereas the core of a single-mode optical fiber typically has a diameter of between about 6 microns and 9 microns. Multimode systems are generally less expensive than single-mode systems, because multimode light sources, connectors, and maintenance can be obtained at a lower cost
Multimode optical fibers are commonly used for short-distance applications requiring a broad bandwidth, such as local networks or LAN (local area network). Multimode optical fibers have been the subject of international standardization under the ITU-T G.651.1 recommendations, which, in particular, define criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The ITU-T G.651.1 standard (July 2007) is hereby incorporated by reference in its entirety. In addition, the OM3 standard has been adopted to meet the demands of high-bandwidth applications (i.e., a data rate higher than 1 GbE, such as 10 gigabits per second (Gb/s)) over long distances (i.e., distances up to about 300 meters). The OM3 standard is hereby incorporated by reference in its entirety. With the development of high-bandwidth applications, the average core diameter for multimode optical fibers has been reduced from 62.5 microns to 50 microns.
Typically, an optical fiber should have the broadest possible bandwidth to perform well in a high-bandwidth application. For a given wavelength, the bandwidth of an optical fiber may be characterized in several different ways. Typically, a distinction is made between the so-called “overfilled launch” condition (OFL) bandwidth and the so-called “effective modal bandwidth” condition (EMB). The acquisition of the OFL bandwidth assumes the use of a light source exhibiting uniform excitation over the entire radial surface of the optical fiber (e.g., using a laser diode or light emitting diode (LED)).
Recently developed light sources used in high-bandwidth applications, such as VCSELs (Vertical Cavity Surface Emitting Lasers), exhibit an inhomogeneous excitation over the radial surface of the optical fiber. For this kind of light source, the OFL bandwidth is a less suitable measurement, and so it is preferable to use the effective modal bandwidth (EMB). The calculated effective bandwidth (EMBc) estimates the minimum EMB of a multimode optical fiber independent of the kind of VCSEL used. The EMBc is obtained from a differential-mode-delay (DMD) measurement (e.g., as set forth in the FOTP-220 standard).
An exemplary method of measuring DMD and calculating the effective modal bandwidth can be found in the FOTP-220 standard, which is hereby incorporated by reference in its entirety. Further details on this technique are set forth in the following publications, each of which is hereby incorporated by reference: P. F. Kolesar and D. J. Mazzarese, “Understanding Multimode Bandwidth and Differential Mode Delay Measurements and Their Applications,” Proceedings of the 51st Int'l Wire and Cable Symposium, 2002, pp. 453-460; and Doug Coleman and Phillip Bell, “Calculated EMB Enhances 10 GbE Performance Reliability for Laser-Optimized 50/125 μm Multimode Fiber,” Corning Cable Systems Whitepaper (March 2005).
FIG. 1 shows a schematic diagram of a DMD measurement according to the criteria of the FOTP-220 standard as published in its TIA SCFO-6.6 version of Nov. 22, 2002. FIG. 1 schematically represents a part of an optical fiber (i.e., an optical core surrounded by an outer cladding). A DMD graph is obtained by successively injecting into the multimode optical fiber 20 a light pulse 21 having a given wavelength λ0 with a radial offset between each successive pulse. The delay of each pulse is then measured after a given length of fiber L. Multiple identical light pulses 21 (i.e., light pulses having the same amplitude, wavelength, and frequency) are injected with different radial offsets 24 with respect to the center 22 of the multimode optical fiber's core. The injected light pulse is depicted in FIG. 1 as a black dot on the optical core of the optical fiber. In order to characterize an optical fiber with a 50-micron diameter, the FOTP-220 standard recommends that individual measurements be carried out at radial offset intervals of about two microns or less. From these measurements, it is possible to determine the modal dispersion (i.e., from DMD graph 23) and the calculated effective modal bandwidth (EMBc).
The TIA-492AAAC-A standard, which is hereby incorporated by reference in its entirety, specifies the performance requirements for 50-micron-diameter multimode optical fibers used over long distances in Ethernet high-bandwidth transmission network applications. The OM3 standard requires, at a wavelength of 850 nanometers, an EMB of at least 2,000 MHz·km. The OM3 standard assures error-free transmissions for a data rate of 10 Gb/s (10 GbE) up to a distance of 300 meters. The OM4 standard requires, at a wavelength of 850 nanometers, an EMB of at least 4,700 MHz·km to obtain error-free transmissions for a data rate of 10 Gb/s (10 GbE) up to a distance of 550 meters. The OM4 standard is hereby incorporated by reference in its entirety.
In a multimode optical fiber, the difference between the propagation times, or group delay times, of the several modes along the optical fiber determine the bandwidth of the optical fiber. In particular, for the same propagation medium (i.e., in a step-index multimode optical fiber), the different modes have different group delay times. This difference in group delay times results in a time lag between the pulses propagating along different radial offsets of the optical fiber.
For example, as shown in the graph 23 on the right side of FIG. 1, a time lag is observed between the individual pulses. This FIG. 1 graph depicts each individual pulse in accordance with its radial offset in microns (y-axis) and the time in nanoseconds (x-axis) the pulse took to pass along a given length of the optical fiber.
As depicted in FIG. 1, the location of the peaks along the x-axis varies, which indicates a time lag (i.e., a delay) between the individual pulses. This delay causes a broadening of the resulting light pulse. Broadening of the light pulse increases the risk of the pulse being superimposed onto a trailing pulse and reduces the bandwidth (i.e., data rate) supported by the optical fiber. The bandwidth, therefore, is directly linked to the group delay time of the optical modes propagating in the multimode core of the optical fiber. Thus, to guarantee a broad bandwidth, it is desirable for the group delay times of all the modes to be identical. Stated differently, the intermodal dispersion should be zero, or at least minimized, for a given wavelength.
To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the optical fiber to its interface with a cladding (i.e., an “alpha” core profile). Such an optical fiber has been used for a number of years, and its characteristics have been described in “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of the above-referenced articles is hereby incorporated by reference in its entirety.
A graded-index profile (i.e., an alpha-index profile) can be described by a relationship between the refractive index value n and the distance r from the center of the optical fiber according to the following equation:
  n  =            n      1        ⁢                  1        -                  2          ⁢                                    Δ              ⁡                              (                                  r                  a                                )                                      α                              
wherein,
α≧1, and α is a non-dimensional parameter that is indicative of the shape of the index profile;
n1 is the maximum refractive index of the optical fiber's core;
a is the radius of the optical fiber's core; and
  Δ  =            (                        n          1          2                -                  n          0          2                    )              2      ⁢              n        1        2            
where n0 is the minimum refractive index of the multimode core, which may correspond to the refractive index of the outer cladding (most often made of silica).
A multimode optical fiber with a graded index (i.e., an alpha profile) therefore has a core profile with a rotational symmetry such that along any radial direction of the optical fiber the value of the refractive index decreases continuously from the center of the optical fiber's core to its periphery. When a multimode light signal propagates in such a graded-index core, the different optical modes experience differing propagation mediums (i.e., because of the varying refractive indices). This, in turn, affects the propagation speed of each optical mode differently. Thus, by adjusting the value of the parameter α, it is possible to obtain a group delay time that is virtually equal for all of the modes. Stated differently, the refractive index profile can be modified to reduce or even eliminate intermodal dispersion.
In practice, however, a manufactured multimode optical fiber has a graded-index central core surrounded by an outer cladding of constant refractive index. The core-cladding interface interrupts the core's alpha-index profile. Consequently, the multimode optical fiber's core never corresponds to a theoretically perfect alpha profile (i.e., the alpha set profile).
The outer cladding accelerates the higher-order modes with respect to the lower-order modes. This phenomenon is known as the “cladding effect.” In DMD measurements, the responses acquired for the highest radial positions (i.e., nearest the outer cladding) exhibit multiple pulses, which results in a temporal spreading of the response signal. Therefore, bandwidth is diminished by this cladding effect.
Multimode optical fibers are commonly used for short-distance applications requiring a high bandwidth, such as local area networks (LANs). In such applications, the optical fibers may be subjected to accidental or otherwise unintended bending, which can give rise to signal attenuation and modify the mode power distribution and the bandwidth of the optical fiber. It is therefore desirable to achieve multimode optical fibers that are unaffected by bends having a radius of curvature of less than 10 millimeters.
One proposed solution involves using a 10 GbE source and injection conditions (i.e., into the optical fiber) that enable transmission that is unaffected by bends. Using such a source and such injection conditions, however, may not always be possible.
U.S. Patent Application Publication No. 2008/0166094, International Publication No. WO 2008/085851, and U.S. Pat. No. 7,787,731, each of which is hereby incorporated by reference in its entirety, describe a multimode optical fiber in which a buried trench is added to the cladding of the optical fiber to reduce or eliminate bending losses. The bending to which the optical fiber is subjected does not generate any attenuation of the signal. However, the position and the depth of the trench must be selected carefully in order not to degrade the bandwidth.
The publication, “Low Bending Sensitivity of Regular OM3/OM4 in 10 GbE Applications” by D. Molin and P. Sillard, Proc. Optical Fiber Communication Conference, JThA55 (2010), which is hereby incorporated by reference in its entirety, defines the system margin M for a system including an optical fiber. The system margin M depends on the effective modal bandwidth and, for a given modal bandwidth, describes the loss of optical energy of the incident signal caused by (i) modal dispersion in the optical fiber or (ii) the time shifts of the higher order modes. This publication discloses an optical fiber in which the modal bandwidth is increased by bending the optical fiber, thus improving the system margin M. This publication, however, does not describe an optical fiber that is insensitive to bending.
European Patent No. 2,166,386 (and its counterpart U.S. Patent Publication No. 2010/0067858), each of which is hereby incorporated by reference in its entirety, describe a multimode optical fiber that is insensitive to bending and includes a buried trench. The optical fiber presents a bandwidth that does not vary with the bending to which it is subjected, because all modes are resistant to bending. The insensitivity to bending, however, is obtained for an optical fiber having a large trench volume. A large trench volume implies deleterious effects on the optical fiber. Large quantities of dopants are necessary to obtain a wide trench, which complicates fabrication of the optical fiber. Moreover, the trench interferes with the bandwidth and guides leakage modes, thereby reducing the quality of the transmitted signal. Those effects can be compensated only by an optical fiber profile that is highly complex.
U.S. Patent Application Publication No. 2009/0010596, which is hereby incorporated by reference in its entirety, describes a multimode optical fiber in which the higher order modes are filtered to achieve transmission that is insensitive to bending. Filtering the bending losses, however, may lead to a loss of quality of the transmitted optical signal.
Therefore, there is a need for a multimode optical fiber that enables transmission without bending-induced degradation of the optical signal and that is free of the disadvantages associated with a large-volume trench.